Airbags have helped save thousands of lives and have helped reduce the number of serious injuries from collisions. In a vehicle equipped with airbags, crash sensors detect an impact. If the impact is sufficiently severe, the airbags are deployed to fill the space between the vehicle interior and the occupant. In this way, the velocity with which the occupant interacts with the vehicle interior is reduced, thereby helping to protect the head and torso of the occupant. In some rare circumstances, however, a vehicle occupant can be injured or even killed by an airbag. For example, if an occupant is not properly positioned, the airbag can cause serious injury or death. Improper positioning can occur with small passengers, such as infants or children. In addition, improper positioning can occur when a passenger is not wearing a seat belt.
To reduce the incidence of airbag-related injuries and deaths, some airbag deployment systems use occupant detection systems to identify whether an occupant is too small for safe airbag deployment. New standards from the National Highway Transportation Safety Administration (NHTSA) require all United States automakers to begin implementing occupant detection systems by 2004. All new vehicles sold in the United States during and after the 2006 model year must be equipped with occupant protection safety features.
Occupant detection systems employ sensors that generate sensor signals. These sensor signals are typically modulated as a function of the occurrence or non-occurrence of an event to be detected. For example, an occupant detection system may incorporate an infrared or other sensor that generates a sensor signal that is modulated as a function of whether a person is within a specified range of the sensor. The occupant detection system then demodulates the sensor signal to identify the presence of an occupant. In some occupant detection systems, the sensor signal may indicate certain properties of the detected occupant, such as weight. Such additional information can be used to inform a decision of whether to enable or disable an airbag deployment system. For example, one type of passive occupant detection system uses a pressure sensor, an electronic control unit, and a seat belt tension sensor to characterize the occupied status of the passenger seat. The data supplied by the sensors is used to classify the occupant, if any, of the passenger seat as an adult or a child or infant. The electronic control unit then communicates to an airbag control system to allow or inhibit passenger airbag deployment.
Some conventional occupant detection systems demodulate sensor signals using an analog synchronous demodulator circuit. In one conventional system, for example, sensor signals are subjected to initial conditioning, including bandpass filtering, amplification, AC coupling, and DC referencing. The conditioned sensor signals are provided to analog peak follower circuits. The peak follower circuits are digitally controlled to enable them to capture the peak amplitudes of the sensor signals. The captured peak signals are further conditioned by difference amplifiers to remove the DC reference. The conditioned peak signals are then amplified and converted to digital signals by an analog-to-digital converter (ADC).
To promote safety, the demodulator circuit should accurately detect peak amplitudes of sensor signals. Failure to accurately detect these peak amplitudes may cause false positives or false negatives. In airbag deployment systems, these errors may cause an airbag to be inappropriately deployed or inappropriately inhibited. For example, if the occupant detection system falsely classifies an adult passenger as a child, the airbag may fail to deploy in the event of a crash. Conversely, if the occupant detection system falsely classifies a child passenger as an adult, the airbag may deploy when such deployment may be hazardous to the child passenger.
In an analog synchronous demodulator circuit, the accuracy of the peak detection depends on the bandwidth of the amplifiers. Using amplifiers with a wide bandwidth may improve the accuracy of the analog synchronous demodulator. Such amplifiers are relatively expensive, however.